Photovoltaic solar cells are semiconductor devices which convert sunlight into electricity. Current applications rely largely on single crystal silicon solar cells. Such solar cells have proven to be too costly for general commercial use. The principal reasons for this high cost are the expense of single crystal silicon, the cost of forming this single crystal (or large grain polycrystalline) silicon into sheets and the absence of high throughput continuous manufacturing processes.
Significant cost reductions can be achieved by using thin-film solar cells. Thin-film solar cells are made by depositing or growing thin films of semiconductors on low cost substrates. These thin-film devices can be designed to reduce consumption of semiconductor material by more than 80%. Design requirements for thin-film cells are provided by A. M. Barnett, et al., "Thin Film Solar Cells: A Unified Analysis of Their Potential," IEEE Transactions on Electron Devices, Volume ED-27, Number 4, April 1980, pages 615 to 630.
Development of thin-film solar cells has been inhibited by problems relating to micro- and macro-scopic defects and to fabrication techniques. Thin-film solar cells are generally polycrystalline in nature. That is, the semiconductor layers are comprised of small crystallites. Where crystallites adjoin, there are crystallographic imperfections, known in the art as grain boundaries. Grain boundaries possess properties which are different from bulk crystal properties, including their electrical and chemical properties. Grain boundaries are known to be the cause of shunts and shorting effects which degrade open circuit voltage and fill factor, recombination which degrades short circuit current, and interdiffusion which degrades reliability and stability.
Another problem confronting the development of thin-film solar cells using polycrystalline semiconductors is the occurrence of macroscopic defects such as pin-holes, voids and cracks. An electrical short occurs when there is a pin-hole in the semiconductor layers of the solar cell and the front and back electrical contacts touch. Such macroscopic defects severely limit performance and manufacturing yield.
Still another problem is the fabrication of thin-film solar cells. Thin-film solar cells are made by sequentially growing the semiconductor layers over a substrate which includes electrical contact means and, for some solar cell designs, optimized light transmission and reflection features. Effective methods of growing semiconductor thin-films on substrates have been limited by contamination of the semiconductor growth environment by the substrate, interdiffusion and chemical reaction between the substrate and semiconductor during growth, degradation of substrate electrical and optical properties during growth, the inability to control nucleation and grain size of the semiconductor layers, and shunts and shorts caused by the grain boundaries.
Solutions to some of the aforementioned problems are known in the photovoltaic art. One solution to macroscopic defects, described in U.S. Pat. No. 4,251,286 issued Feb. 17, 1981 to A. M. Barnett, is selectively forming an insulator or appropriate semiconductor material which effectively blocks shorts and shunts that are caused by macroscopic defects in the semiconductor layers of thin-film solar cells.
D. E. Carlson, et al. indicate in United States Department of Energy Report No. SAN 1286-8, entitled "Amorphous Silicon Solar Cells, Final Report For the Period July 1, 1976 to Sept. 30, 1978 Under Contract No. EY-76-C-03-1286", Oct. 1978, pp. 22-24, that electrical shorts can be eliminated by the use of resistive films having a thickness equal to or greater than that of the semiconducting film. The Report describes alleviating the problem of shorts due to pin-holes by use of a thick back-cermet ballast resistor such as Ni--SiO.sub.2.
Solutions to microscopic defects associated with grain boundaries include selectively depositing an insulating cap at the surface intersection of the grain boundaries, described in U.S. Pat. No. 4,197,141 issued Apr. 8, 1980 to C. O. Bozler, et al. However, this approach does not eliminate the adverse effects of grain boundaries within the semiconductor layers. U.S. Pat. No. 4,366,338 issued Dec. 28, 1982 to G. W. Turner, et al. describes electrically passivating grain boundaries in p-type GaAs by introducing tin as an n-type compensating dopant into the interstices of p-type grain boundaries. However, the grain boundary passivation approach of Turner, et al. has not proven to be effective.
Solar cells which are deposited on a substrate are described in U.S. Pat. No. 3,914,856 issued Oct. 28, 1975 to P-H. Fang. U.S. Pat. No. 3,914,856 teaches evaporating an aluminum metal contact electrode on a flexible substrate and depositing a thin layer of crystalline silicon. According to the patent, the aluminum substrate is used for a nucleation site for growth of the silicon crystals and for autodoping of the silicon. Such a solar cell embodies all of the aforementioned disadvantages that have inhibited development of thin-film solar cells. U.S. Pat. No. 3,914,856 also mentions introducing a silicon oxide layer to the substrate before the metallic electrode evaporation as an additional step. The silicon oxide layer is described as serving three purposes: electrical insulation from the substrate; reduction of diffusion between the substrate material and the semiconductor; and better matching of the substrate for growing silicon films.
U.S. Pat. No. 3,961,997 issued June 8, 1976 to T. L. Chu describes preparation of polycrystalline silicon solar cells by depositing successive layers of doped silicon on steel substrates which are coated with a diffusion barrier of silica, borosilicate or phosphosilicate.
A problem inherent in the solar cells of the U.S. Pat. Nos. 3,914,856 and 3,961,997 is that layers of silica, silicon oxide, etc., are electrical insulators. U.S. Pat. No. 3,914,856 does not describe electrical contact means in such solar cells. U.S. Pat. No. 3,961,997 describes placing ohmic contacts in the n- and p- regions of the device on the light receiving top surface of the silicon. Thin-film solar cells with both electrical contacts on the front surface of the silicon have the disadvantages of increased cost and unacceptable losses in performance.
Some of these problems associated with growth of thin-film semiconductors on substrates can be overcome by the metallurgical barrier layers described in U.S. Pat. No. 4,571,448, issued Feb. 18, 1986 to A. M. Barnett. This patent describes barrier layers, such as silicon carbide or tin oxide. Such barrier layers serve several useful functions: prevent contamination and diffusion during semiconductor growth; electrical communication between the substrate and semiconductor layers; and enhanced optical reflection for increased efficiency. However, metallurgical barrier layers do not overcome problems associated with macroscopic defects or microscopic defects such as grain boundaries.
Applicant has recognized that these obstacles to developing high efficiency, low cost thin-film solar cells can be surmounted by providing an improved substrate for thin-film solar cells. Accordingly, an object of this invention is to provide a solar cell which includes a novel substrate having an insulator and electrically conductive nucleation sites. Another object of this invention is to provide methods for making improved thin-film solar cells which include fabrication of a novel substrate.